1. Field of the Invention
The present invention relates to an optical surface defect inspection apparatus and an optical surface defect inspection method, and more particularly to an optical surface defect inspection apparatus and an optical surface defect inspection method preferable for use in detecting line-shaped micro defects (scratches) produced on the surface of a test subject.
2. Description of the Related Art
Optical surface defect inspection apparatuses for use in detecting micro defects on the surface of a test subject such as a magnetic disk and an IC wafer are demanded to perform high speed inspection that can meet a 100-percent inspection of the entire surface of all test subjects as well as highly sensitive inspection (detection of micro defects having a width of about a few tens nanometers and a depth of about a few nanometers). Line-shaped micro defects (scratches) considerably damage products particularly, and it is essential to inspect these defects. Generally for highly sensitive defect detection, such a method is adopted that a high intensity micro spot is applied to the surface and the micro spot is scanned over the surface for detecting a scattered light from a defect on the surface highly sensitively. It is necessary to coarsen scan pitches for quick completion of overall scanning in order to perform high speed inspection. In this case, however, the size of an applied spot is needed to sufficiently cover at least the scan pitch. However, there is a dilemma that an increased spot size reduces spot intensity and decreases detection sensitivity.
For a method of performing a highly sensitive surface defect inspection at high speed (Japanese Patent Application Laid-Open Publication No. H02-061542), there is a method that a bundle fiber is formed in a multi-segmented cell structure in which the bundle fiber is divided into n small segments, the bundle fiber has a scattered light detector disposed in the radial direction of a test subject, an image in a micro visual field on a micro spot applied to the surface of the test subject is formed on the multi-segmented cells, and the level of background noise per cell (noise mainly caused by surface roughness) is made 1/n. Moreover, in the method described in Japanese Patent Application Laid-Open Publication No. H02-061542, in order to eliminate position dependence in the radial direction, the individual small segments are arranged uniformly in the radial direction. A defect signal in this case is detected at one place in any cells corresponding to the position of the defect. On the other hand, background noise is reduced by 1/n because the cell area is decreased to 1/n of the entire area. Consequently, the level of a cell signal that detects the defect is not made smaller, whereas only noise becomes 1/n, resulting in an increase in the signal-to-noise ratio n times. Furthermore, since the scan pitch can be widened to the width of n cells of the detection cells, inspection speed is not reduced.